This invention relates to one-part, room-temperature latent, curable liquid isocyanate compositions. More particularly, it relates to such compositions wherein a trimerization catalyst solution is encapsulated by a shell wall comprising partially polymerized isocyanate.
Isocyanurate resins and catalysts for forming the same are known in the art, see Saunders and Frisch, Polyurethanes: Chemistry and Technology; Part I, pp. 94-97, Interscience Publishers, New York (1962). Examples of known isocyanate trimerization catalysts include tertiary amines, alkali metal hydroxides, alkali and alkaline earth metal salts of carboxylic acids, metal alkoxides, xanthate salts, quaternary ammonium hydroxides, etc.
Known catalysts for trimerizing isocyanates can be divided into two general classes. First, those that cause trimerization of isocyanate exothermally at room temperature and second, those that trimerize isocyanates only at elevated temperatures and prolonged reaction times. The first group lacks latency and therefore may be used only in two-part compositions. The second group generally has a short shelf life (e.g. several weeks) and requires long cure times (e.g. several hours at elevated temperatures).
Curable resin compositions containing encapsulated catalysts are known. Examples of such compositions include the polyepoxide compositions containing capsules having a hardened, inactive colloid shell wall (e.g. gelatin) about a curing agent that are disclosed in U.S. Pat. No. 3,018,258. Such compositions are substantially unreactive under normal temperatures, having shelf life of several months, but require prolonged heating (e.g. 24 hours) in order to cure. U.S. Pat. No. 3,455,848 discloses stable compositions that contain an intimate mixture of polyurethane components, at least one of the components being contained in capsules having a shell wall of gelatin. On heating the composition, the encapsulated component reacts with the unencapsulated component or components to form a polyurethane.
U.S. Pat. No. 3,467,544 discloses resin compositions comprising an encapsulated polysulfide. A gelatin capsule wall contains a metal oxide catalyst suitable for curing the polysulfide. Upon rupture of the capsules, polysulfide is released contacting the catalyst and curing takes place.
U.S. Pat. No. 3,505,254 discloses polysulfide compositions containing encapsulated metal oxide, which compositions cure upon breaking of the capsule shell walls. The shell walls can be lignin sulfonates, phenol, formaldehyde penterythritrol esters, etc.
Processes for microencapsulation of materials are also known. A general microencapsulation reference is Capsule Technology and Microencapsulation, Noyes Data Corp., Park Ridge, N.J. (1972). In these processes a polymer is caused to separate from a continuous liquid phase and form a solid shell wall about liquid or solid particulate material suspended in the continuous phase. There are several general microencapsulation processes for accomplishing this. In one such process, known as "simple coacervation," a polymer separates from a solvent solution of the polymer by the action of a precipitating agent that reduces the solubility of the polymer in the solvent (e.g. a salt or a nonsolvent for the polymer). Patents describing such processes and their shell wall material include U.S. Pat. Nos. 2,800,458 (hydrophilic colloids); 3,069,370 and 3,116,216 (polymeric zein); 3,137,631 (denatured proteins); 3,418,250 (hydrophobic thermoplastic resins); and others.
In another such process, known as "complex coacervation" a polymer having an electric charge of one polarity is caused to separate from solution by the addition of another polymer having the opposite polarity, whereby the first polymer is encapsulated by the second polymer. Patents describing such processes include U.S. Pat. Nos. 2,800,457 (complex colloid); 3,317,434 (gelatin); 3,324,065; and others.
In yet another encapsulation process, a shell wall is formed about suspended particulate material by an interfacial polycondensation reaction. In this process, the material to be encapsulated is mixed with a solution of one component of a polycondensation reaction and the mixture dispersed in a non-solvent for the mixture. A second component of the polycondensation reaction is then added to the resulting dispersion. The subsequent reaction forms a shell wall of polycondensate about each dispersed droplet at its interface. Patents describing such processes include U.S. Pat. Nos. 3,427,250; 3,565,818; 3,575,882; 3,577,515; 3,594,328; and others.
In a variation of the interfacial polymerization process, encapsulation is accomplished by an interfacial addition polymerization. In this process, particulate material is suspended in a solution of a polymerizable, ethylenically unsaturated compound in a solvent. This solvent is not a solvent for the polymer formed from the unsaturated compound. The unsaturated compound is polymerized such as by the use of free radical initiators. The resulting polymer forms a shell wall about the suspended particulate material. Patents describing such processes include U.S. Pat. No. 3,427,250 and others.
None of these patents disclose one-part, room-temperature latent, curable isocyanate compositions that have prolonged storage stability and a rapid cure.
However, in U.S. Pat. No. 3,860,565 (Barber) there is disclosed a one-part, room-temperature latent, curable isocyanate-catalyst system that comprises an aromatic polyisocyanate material having dispersed therein an encapsulated liquid isocyanate trimerization catalyst comprising a viscous solution of an alkali metal salt of an organic acid in a lower polyhydric alcohol. The liquid trimerization catalyst is encapsulated by a shell wall comprising partially polymerized isocyanate that is impermeable to said catalyst at room temperature but permeable to said catalyst at about 75.degree.C.
The system disclosed in U.S. Pat. No. 3,860,565 requires from 30 minutes to one hour before a shell wall forms which is impermeable to the catalyst at room temperature. Moreover, that patent discloses systems wherein the catalyst solution may comprise up to about 30 percent by weight of the system during encapsulation. The concentration of the encapsulated catalyst solution is then preferably reduced to from about 0.1 to 5 percent by weight of the system by dilution with additional polyisocyanate. The resulting systems have good shelf stability.
The present invention is an improvement upon the prior art. It provides one-part, room-temperature latent, curable isocyanate compositions which can contain up to 50 percent by weight encapsulated catalyst and which can also be diluted with a wide variety of polyisocyanates and still have good shelf stability at room temperature.